In steel pistons for an internal combustion engine, in particular for diesel engines, the problem arises that the burning fuel cloud, ignited after the injection, generates a very large amount of heat. The combustion chamber bowl, which is constructed in the crown of the steel piston, is thereby subject to an intensive oxidation, typically in regions close to and/or on the upper edge of the combustion chamber bowl, i.e. the lip-like boundary region between the combustion chamber bowl and the flat upper edge of the piston crown, i.e. the bowl edge. In this oxidation, the iron in the steel is oxidised to Fe2O3 and the resulting oxides have no adhesion to the non-oxidised steel material of the piston lying therebeneath.
Through mechanical expansion/contraction processes, the oxidised layer which is formed is finally detached, with so-called scale notches being formed. Through this process, the regions which are thereby eroded become so large over the course of time that they are visible to the naked eye. Through further spreading of these scale notches into the base material of the steel piston, finally bowl edge cracks can occur, which in extreme cases can lead to a destruction of the piston. Furthermore, owing to the scale notches, the problem arises that through the corresponding change to the form of the bowl edge, disturbances of the combustion process can be brought about, and thereby the exhaust emission characteristic of the engine is impaired.
With regard to the problem described above, a method is known for example from U.S. Pat. No. 7,458,358 B2, in which a coating material is applied onto the piston crown of a piston for an internal combustion engine, which coating material has a microstructure and a porosity, wherein the coating is irradiated with a high-energy laser beam, in order to increase the density of the coating, whilst at the same time the microstructure is converted and a material connection is produced between the coating and the piston crown surface, and a portion of the coating is masked, in order to prevent an irradiation with the laser beam.
From EP 1 217 095 A1 a protective coating is known for a thermally stressed component, in particular a turbine component, for protection from corrosion and/or oxidation and/or erosion, wherein the protective coating has a single-layered or multi-layered sealing coating of an amorphous material.
In addition, from DE 197 41 800 A1 a layer system is known for coating metal substrates which are liable to corrosion with at least one intermediate layer and at least one functional layer, wherein the functional layer is a nitride, carbon nitride and/or oxynitride of at least one of the metals from the 4th to 6th subgroup of the periodic table, and the intermediate layer is composed of one or more metal oxides.
With the coatings and methods of the prior art, however, the above-mentioned formation of scale notches and the damage to the piston entailed therewith cannot be completely satisfactorily prevented.